DovLK — Some Researches in Experimental Morphology. 429 



mean, however, tliat permanent tissue can be induced to become active and to 

 give rise to liighly differentiated vascular elements, if an excess of foodstuff 

 be present. Tliis is contrary to the usually accepted view of the relation 

 between nutrition and development as expressed for example by Jost (24, 

 p. 447), " wir siud der Meinung dass er [i.e. foodstuff] durch das Waehstum 

 und den Verbraucli Bedingtes und nicht die bedingende Ursache der 

 Qestaltung ist." But that this is not universal is shown by Voohting's 

 experiments of producing tubers abnormally in Bomsimjcmltia and on 

 petioles of Raphanus. He himself attributes tlii.s development to a nutritive 

 stimulus. But the result of tliat stimulus is storage parenchyma, and not 

 vascular tissues. It develops tubers, and not masses of tracheides. If 

 now we lookback again at the peculiar periderm on the underground portion 

 of our rooted leaves, we have sucli parenoliyma immediately suggested to us. 

 But Plate XXXIV, fig. 2, shows — I think indubitably — a tuberous nature. 

 Again, if the Plumule is removed from a growing seedling of Phaseolus 

 multiflorus, the hypoeotyl enlarges greatly, the cambium having given rise to 

 thick layers of parenchyma — this parenchyma being full of starch. There is 

 also a large amount of reserve starch deposited in the medullary cells, such 

 reserve being derived from tliat originally present in the seed, since the embryo 

 has none, and the experiment was conducted in the dark (Jost 27 and 28). 

 Here again, we liave storage parenchyma as the result of nutritional stimulus. 

 It may be objected tliat Wieler and Hartig maintain that a nutritional 

 stimulus is the origin of the secondary growth of trees. Wieler's works 

 I have been unable to obtain, but Hartig only claims as a result of 

 nutrition the different thicknesses of the cell-walls of the tracheae and 

 traclieides at different seasons. He does not claim tliat vascular tissue 

 owes its origin to a nulriti<mal stimulus (29). Until, then, we liave evidence 

 that storage of food material can induce the appearance of secondary 

 vascular elements we can decide that this factor is of no importance in 

 the development of the secondary tissue in our petiole. 



The next factors cannot be disposed of so easily. Indeed they are also 

 so intimately connected that they must be taken together — they are the 

 stimulus of increased mechanical strain, and the stimulus of increased conduction. 

 If we glance again at Plate XXIX, figs. 2 and 3, we realize this very 

 clearly. Plate XXIX, fig. 2, shows us the heavy weight of foliage which 

 must be borue by the petiole; tiie strains and stresses to which it is subjected 

 must be enormously increased. It shows, too, tliat a great activity of 

 assimilation is in progress, and that thus a large transpiration current 

 and a large stream of plastic substances must be passing in the petiole 

 between this foliage and a large root-system such as in Plate XXIX, 



SCIENT. PKOC. K.D.S., VOL. XIV., NO. XXXIII. 3 Z 



